.. The author of a text book, Henslin,  wanted to make observations about homeless people. . His method was participant observation. . He wanted to live with them and to experience what they experienced, and to find out what they themselves thought about the meanings of what they were doing. . He could not live with more than one homeless group at a time, so his sample consisted of only one community among the thousands that exist throughout North America and the rest of the world. . If we knew for sure that every homeless community was radically different from every other one, then the validity of his findings would be very low. . If we believe, as we do, that there are many similarities between those communities, his validity would be very high, much higher, for example, than if he went around with a clipboard asking questions from a questionnaire. . (See that the method and the sampling technique are not as unrelated as one might first imagine both in combination contribute to the level of validity). . The text book gives an example of a population being all the students of a university, and a sample being those interviewed with a questionnaire. . How do you choose that sample? . Do you ask every fifth person walking down a particular hallway? . Will that bias your sample towards students taking those subjects taught in the rooms in that hallway? . Would that bias affect the results? . If you were asking questions where there might be significantly different answers from students taking one subject, rather than another, then that method of sampling would bias the result. . He suggests that a random sample would be most valid. . So how do you choose a sample that is random? . Here you might exercise your imagination, initiative and creativity. . How much a proportion of the population should the sample be so that it is valid? . If you interview only ten people out of a population of several thousand, perhaps the results would not be as valid as it might be if you interviewed a sample of one thousand. . My PhD research was based mainly on participant observation.  See Why Obo? .

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